A semiconductor laser (laser diode) transforms electrical energy into optical energy with relatively high efficiency. A laser diode is typically includes a layer of p-type semiconductor material adjacent to a layer of n-type semiconductor material (referred to as a p-n junction). When electrical current passes from the p-type layer to the n-type layer, stimulated emission of optical radiation results in the active layer. In practice, the stimulated emission is limited to only a portion (the active region) of the active layer. The opposing end faces of the active region are called the facets, which are cleaved and/or etched to define a laser cavity between the two facets. A highly reflective dielectric coating is usually deposited on one facet (the non-output facet), and a semi-reflective dielectric coating on the other facet (the output coupling facet). The optical energy generated by the electric current oscillates between the output facet and the non-output facet, and is partially transmitted by the semi-reflective coating at the output facet to produce a diode laser output beam.
Laser diode bars are constructed from a linear array of such individual laser diodes, with all the diodes typically driven in parallel from two highly conducting electrodes. Stacks of these bars can then be driven in series to form a laser diode array, which is a two dimensional array of individual diodes.
Diode junction aging, associated degradation, and catastrophic failure are serious problems in laser diodes. One failure mode of laser diodes is catastrophic optical damage (COD), which occurs suddenly after more gradual diode aging in which the performance of the diode degrades slowly with time. Gradual aging is a result of localized junction overheating caused by filamentation of the diode current and of the output optical beam. Initially, current and optical filamentation of the diode current is caused by local variations in the electrical and optical properties along the diode junction. For example, variations in electric field along the junction result in local current variations and also in local optical laser beam intensity variations along the diode output. These variations in electric field and associated current density variations also lead to temperature variations along the junction. Small changes in the local electric field (2% relative to the average electric field across in the junction), for example, can lead to large changes in the local current density (relative to the average current density in the junction) and temperature, and therefore large changes locally in the intensity of the optical laser beam. Gradual aging, resulting from these current density and temperature variations, culminates in catastrophic optical damage (COD) and/or catastrophic optical mirror damage (COMD). COD and COMD are caused by an instability which rapidly leads to catastrophic overheating and results in the failure of that portion of the diode junction. COD and COMD result from destructive overheating of the junction material and/or the diode facet or coating material.
Multiple modes of laser diode failure arise from filamentation of the drive current to the diode or the diode bar, or filamentation of the optical beam within the laser active medium (the active portion of the p-n junction). These modes can range from overheating and destruction of the output facet, migration of dopants, and junction punch through.
In laser diodes, high current density must be provided in order to reach lasing threshold, and even higher current densities are needed to reach optimum laser output efficiency, laser power, and laser brightness. However, even if the laser is driven by a so-called constant current source, the current can filament in a region or regions of the active junction resulting in some sections of the junction experiencing higher current density than others. With a constant current source, these regions of higher current density have lower impedance to current flow than surrounding regions which experience lower current density than average. It is the sections of the junction experiencing higher current density that have higher temperature, age more rapidly, and are prone to unstable filamentation instabilities, In cases where the current is filamented in the diode, due for example to variations of electric field across the junction, the total current in the diode must be adjusted so that the sections of higher current density do not result in unacceptably rapid aging. However, accommodating these regions of higher current density in this manner reduces the efficiency and intensity in sections of lower current density, and therefore the overall efficiency and power of the diode is ultimately compromised. Since the bandgap of the semiconductor material changes with temperature, filamentation also leads to shifts and spreads of the output spectrum of the laser diode. These shifts and spreads in the output spectrum can reduce the efficiency of coupling to the pump bands of solid state laser media pumped by these laser diodes. Efficiency is defined as optical power output divided by electrical power input.
Initially, electric field variations and resulting current filamentation can lead to large, but stable changes in the local diode junction temperature and in the intensity of the output laser beam. Later, as these stable current, temperature, and optical intensity variations age the diode junction, the filamentation can become unstable, and the larger unstable current, temperature, and intensity changes in the region can lead to COD and/or COMD.
Controlling the current density in the junctions of laser diodes, laser diode bars (LDBs) and laser diode arrays (LDAs) is complicated by the fact that the junction bandgap decreases with increasing temperature. In junction regions having higher perturbed electric fields, the current density is higher. In these sections of the laser diode junction with higher current density, the temperature is higher and the bandgap is lower. When the bandgap shrinks, the current density in this section can increase even more at the expense of the current density in adjacent sections (even with a so-called constant current source powering the diode). The increased current density in this section then increases the temperature locally even further, and the bandgap shrinks even further. This instability can continue until the current density and temperature in this section is sufficiently high to cause cumulative incremental damage (aging) and ultimately catastrophic damage (COD and/or COMD). These instabilities can be driven by small variations in electric field across the junction, which can be caused by local changes in the junction material properties or by edge effects at the periphery of the junction. These initial variations can also be caused by crystal defects. The positive feedback process starting with increased current density in regions of higher electric field, leads to locally higher temperature, locally reduced bandgap, and then to even higher local current density. This positive feedback results in rapid thermal runaway, and breakdown locally of the p-n junction. This thermal runaway in the region of current filaments creating “hot spots” is referred to as a current filamentation instability
As detailed above, operation of a semiconductor junction can lead to filamentation instabilities which constrict the current flow through the junction. Filamentation increases the current density and the temperature of the junction locally (where the current is flowing). Because the bandgap for these semiconductor junctions typically decreases as temperature rises, the locally increased junction temperature leads in some cases to even higher current density in that local region. The local increase in current density is not necessarily accompanied by an increase in total current to the device. This feedback mechanism leads to an instability in which the temperature of a section or sections of the junction continue to rise until damage is done to the semiconducting junction or to adjacent structures.
Restricting the total current to the semiconducting junction with so-called current regulation circuits or constant current drive circuits does not prevent filamentation instabilities or their consequences, which include thermal runaway, accelerated aging, and ultimately device failure. Even if the total current to the terminals of the device were held constant, device current can constrict in a local region or regions of the semiconducting junction so that excessive heat is deposited locally in a limited region or regions of the junction. Such localized junction heating along only a portion of the junction leads to local thermal runaway, accelerated device aging and ultimately premature device failure.
Measuring or sensing the external temperature of the device and then interrupting the current to the device when a temperature anomaly is sensed may not be effective in preventing these instabilities and their consequences (thermal runaway, accelerated device aging; and ultimately premature device failure). When current filamentation instabilities occur, the external temperature of the device may rise, fall, or stay the same. So measuring the temperature of the outer surface of the device (the device case) with a temperature sensing device is not necessarily an indicator of these instabilities. Temperatures of interest in catastrophic optical damage of the laser facet are the local temperatures caused by excessive absorption of the optical power at small regions of the junction. Temperatures of interest in catastrophic damage to the junction itself are in the active region of the junction which is a thin layer region (0.1 to 1.0 microns sandwiched between thick layers (50-100 microns) of semiconductor material. Temperature anomalies in local regions of the semiconductor junction take time to diffuse to the device surface. This lag time may be many microseconds or milliseconds. Device damage can occur during this lag time.
In laser diode devices, these current filamentation instabilities can be exacerbated by the nonlinear interaction of the laser beam with the laser gain medium. These Kerr-type instabilities can lead to self-focusing of the laser light within the laser device. This instability can interact with the current instability described above, damaging the diode facets and leading to so-called catastrophic optical damage (COD) and catastrophic optical mirror damage (COMD).
In some semiconductor devices with diode junctions, device function is not the production of light, but to simply switch a voltage or current or to otherwise regulate a voltage or current. These devices are also subject to the current filamentation instability described above. Current instabilities in these devices also lead to thermal runaway, accelerated device aging, and ultimately device failure. High current semiconductor switch manufacturers already use special techniques to mitigate the effects of such current filamentation. Since current density need not necessarily be made high in these devices, high current semiconductor switches sometimes employ geometrical methods for spreading the current in the junction of these devices by using spider-shaped electrode structures and other current spreading geometries and techniques to divide the device current before it is injected into the semiconducting medium. It must be noted that many of the instability mitigation techniques used presently in these semiconductor switch devices cannot be used in geometries required by laser diodes, light emitting diodes and VCSELs where the current density must reach a certain minimum value to reach threshold lasing and then an even higher current density value to optimize laser output, efficiency, and brightness.
Spreading and limiting the current in laser diodes and light emitting diodes using resistive layers before the current enters the semiconducting media has also been attempted. However, the series resistance needed in these layers lead to power dissipation and unacceptable reductions in the overall electrical efficiency of these light sources. In addition, these resistive ballasting techniques ultimately cannot prevent current filamentation in the active junction region which is a thin layer sandwiched in a much thicker semiconducting medium.
Laser diodes, light emitting diodes, and VCSELs are sometimes arranged in bars or arrays. For bars in which multiple devices are driven in parallel, the same type of fault mode mitigation and protection circuitry used for a single device can be effective in suppressing and protecting against instabilities. In a laser diode bar, all of the laser diodes are driven in parallel from the same current node. Physically this current node is typically fabricated from a material with high electrical conductivity and high thermal conductivity such as copper. These current nodes also serve a second function which is to cool the individual diodes by transporting waste heat generated in the diode to a heat sink.
Current instabilities similar to those which occur in single diodes can also occur in laser diode bars. In addition to filamentation within individual diodes in the bar, this instability also causes current hogging, in which the current to the common node for all the diodes in the bar is not shared equally among the diodes in the bar. The diodes that hog more current than the average current (average current=total current to the node/the number of diodes in the bar) are prone to overheating and thermal runaway. Note that such an instability is not prevented by using current regulating circuitry or so-called constant current sources to power the laser diode bar. Although laser diodes were used here as an example, other devices containing semiconducting junctions such as light emitting diodes and VCSELs are also sometimes arranged in parallel in bars, and the same descriptions and conclusions apply to them as well.
Note that current sharing in laser diode bars can be enforced by using separate resistors in series with each diode in the bar. However, true current sharing requires that the value of the series resistance be approximately as large as the effective resistance of the laser diode, so that the electrical efficiency of such a bar would be reduced by a factor of 2 or more.
For arrays of these devices driven in series, current hogging is not an issue, since the same total current must pass through each device. However, current filamentation instabilities within each of these devices can still occur with the consequences that have been discussed above.
The concept of driving the individual diodes making up bars in series rather than in parallel may have beneficial effects since current hogging is not allowed and current sharing is imposed in a series configuration. As discussed above, laser diode bars are now driven in parallel for convenience. It is easier to cool these devices from a common, high thermal conductivity, electrode structure—the highly conducting node described above. However, high thermal conductivity, low electrical conductivity materials such as beryllium oxide and diamond can be used as electrical insulators. With these materials to remove heat from the individual diodes, it is possible to contemplate rearranging diode bars into a series configuration. Such a configuration would ensure that the current through each device in the bar is equal, thereby removing the possibility of current hogging. Such configurations however do not remove the possibility of filamentation.
In some instances, the presence of noise can mask or simulate the effects of filamentation.